1. Field of the Invention
The invention relates to a process and apparatus for forming long and thin sheet products, more specifically, a process and apparatus for continuously forming sheet products in such a way as to produce flat, mirror-finished surfaces or surfaces with specific patterns.
2. Description of Related Art
Long sheet products such as plastic sheets or films (hereinafter may be referred to simply as "plastic sheets") have hitherto been produced by forming molten plastic materials into desired shapes by means of extrusion forming devices.
Such an extrusion forming device is not intended to roll molten plastic materials coming out of a die for the formation, but rather to give a specific surface finish such as mirror-finish to surfaces of plastic sheets having a predetermined thickness extruded from the die. The forming process is, for example, composed of transporting and processing plastic sheets through a series of equipment such as surface finishing rolls, an annealing unit, a take-up roll unit, a transport unit, and an edging machine.
The molten plastic material, after being formed into a plastic sheet of a desired thickness passing through the above-mentioned series of equipment, is cut to size by a cutting machine and stacked, or taken up by a take-up roll.
Various means have been used to adjust the thickness this formed plastic sheet, for example, die lip adjustment, or adjustment of the gap between the surface finishing rolls in case of a thicker sheet.
In case of a thinner sheet, a typical means has been either the air knife method, which is a method of pressing molten film-like plastic materials against the cast drum using pressurized air from a nozzle, the touch roll method, which is a method of adjusting the touch of the rubber roll against the forming roll, or the pressure forming method using a cast drum and an endless metal belt, which is a method of feeding thermoplastic materials between the cast drum and the endless metal belt wherein the plastic materials are sandwiched between the drum and the belt abutting against the drum outer surface in an arc and cooled to be formed into desired shapes (ref. to JP-A-06-166089 and JP-A-06-170919).
However, the air knife method has a problem that air pockets are generated between the molten plastic materials and the forming roll and thus make it difficult to obtain uniform plastic sheets with smooth surfaces.
The touch roll method has a problem that it is difficult to produce sheets with high degree of mirror-finishes or sheets with patterns because, as the surface of the touch roll itself is made of rubber, the roll surface tends to be easily damaged and the damage mark gets copied onto the plastic sheets, or the plastic sheets stick to the rubber roll surface when it gets hot.
The pressure forming method has a problem that, in order to produce plastic sheets with uniform, mirror-finished surfaces, special techniques are needed to smoothen the surface of the joining part of the belt. As a result, the endless metal belt turns out to be expensive and the products become expensive as well.
The metal endless belt is not only expensive, but also long (4-5 (m)) and heavy (20-40 (kg)), so that it has a disadvantage of having to deal with cumbersome replacement and tensioning of the belt when it becomes necessary to replace the belt due to wears, etc. If the product lengths are short and a large varieties of surface patterns have to be produced, i.e., in case of a large-variety-small-lot production, the belt has to be replaced frequently, productivity drops due to the cumbersome belt replacement procedure, and consequently the products becomes more expensive.